He was a Forerunner AI designed to study the Flood. He found the gravemind and began an extremely long conversation with it in which it convinced Mendicant to turn against the Forerunners. He led a flood infested fleet of derelict Forerunner ships in a climactic battle at the Maginot Line. The attack failed largely because the defensive AI sacrificed the crew of all ships and used the Promethean knights as living bombs to destroy infected ships. Mendicant Bias was unable to escape and was fragmented by the Defensive AI whose name I've forgotten. One fragment of his AI was put on the Ark and another was the Oracle the Covenant originally spoke to.